CHAPTER 3

EXPERIMENTAL CONSIDERATIONS

PETER H. KLOPFER AND JEREMY J. HATCH

WHAT IS COMMUNICATION?

The concept of communication has been so enthusiastically embraced by such a diversity of biologists that the term is in imminent danger of being cast aside contemptuously as confusingly broad. Aggregation of slime molds, echolocation of bats, and territorial signaling of birds have all been, by implication, cast under one rubric (e.g., Sebeok, 1965). This makes it difficult to find reasons for excluding many other biological phenomena whether of a cellular, organismic, or societal nature. Whatever can be said for focusing upon the common elements, formal and otherwise, of the diverse systems found upon this planet, there are equally convincing arguments for distinguishing differences.

The most cogent of these arguments is empirical: whatever we learn of the processes called “communicative” at a cellular or subcellular level will teach us little of the nature of communication between whole animals. Neither the evolutionary history, the ontogenetic development, nor the physiological mechanisms of social communication among animals can be predicted from a study of gene-enzyme interactions or comparable molecular phenomena. This is said, not to disparage studies of “communication” at the subcellular or cellular level (hereafter called “interactions”), but to emphasize that analogies are no substitute for analyses. Of course, it is not always clear what constitutes a “whole” animal, for instance among slime molds.

The second argument for differentiating between communicative and interactive phenomena occurring at various levels in the organizational hierarchy rests upon a teleonomic premise. There is a cost connected with communication, and energy used for communication is not available for other purposes. Thus we can reasonably assume that the existence of communication among members of animal societies is due to communication being favored by natural selection. “Being favored” means that the beneficiality or fitness of the communieating members is increased. Fitness is measured by the proportion of any future population which can be attributed to a given individual (Fisher, 1930). However, this is a measure, and hence fitness is a concept, that can be applied only to the reproductive unit, the whole organism. A similar argument applies to the prey-predator relationship, which should also be excluded from discussions of communication.

Some kinds of perceptual processes, e.g., echolocation and the detection of distortions in self-generated electrical fields, are also occasionally thought of as instances of communication. These processes depend upon interactions between the environment and the signals, whereas we propose to limit “communication” to the relationship between individuals. This is not to deny that the different phenomena may share common origins and, indeed, may overlap in function. However, it is questionable whether echolocation et al. are worth distinguishing from other types of perception.

Communication can most conveniently be recognized empirically in terms of the relationship between the communicators signal and the recipient’s response. It is customary to distinguish communication from other forms of causality in terms of the energy incorporated into the signal or, as Cherry puts it (1957), to distinguish between the command “jump in the lake” and a push, both of which may have the same result. Obviously, this distinction is only of limited value but is relevant to the evolution of the signaling system.

In brief, we can say that communication, sensu stricto, necessitates the existence of a code shared between two or more individuals (Smith, 1965) whose use is mutually beneficial to its possessors, i.e., increases fitness.

THE COMMUNICATION PROCESS

The ultimate criterion for recognition of the occurrence of communication is that of a resultant change, sometimes delayed, sometimes scarcely perceptible, in the probability of subsequent behavior of the other communicant. Bearing in mind our earlier restrictions of noncommunicating interactions, the initial requirement for communication is a code shared by the communicants. This term “code” implies more than just a shared repertoire of signals (e.g., movements or sounds), for, as W. J. Smith (1964, 1965) has suggested, the context in which the signal is given alters the meaning; i.e., the sets of rules that compose the code relate to the total communicatory situation. In the everyday sense, the context is the immediate situation at the time of the event which affects, though differently, both communicator and recipient, but the distinction between signal and context is really only an arbitrary one. This context cannot be usefully isolated from the past history of the communicants or from the past of the species; i.e., the phylogeny and ontogeny of the communication are other contexts (see below).

A communicatory system involves a number of components which are represented somewhat after the fashion of communication engineers in the following diagram:

Following the usage of W. J. Smith (1965) we can say that the communicator emits a signal, incorporating the message, which describes the state of that individual in a code held in common with the other communicants, or recipients. The other sensory inputs to the recipient provide the immediate context. The meaning to the recipient is dependent on the context of the signal. The meaning can probably be operationally defined as the response selected by the recipient from all responses open to it [though for humans, Cherry (1957) suggested that the response intended by the communicator constitutes the meaning]. Such a diagram is sometimes misleading, particularly because it tends to oversimplify the nature of the sensory input reaching the recipient from the communicator; e.g., for a distant bird the information from another can be a variety of visual and auditory clues. To make the diagram more nearly complete by including diverse modalities and an indication of contextual input would render it inconveniently complex.

Communication can be studied by observing the behavioral interactions of animals under a variety of conditions. Naturalistic observations of signals and their spatial and temporal context can give some information on the repertoire available to the communicator (see Chapter 1). This repertoire may be limited by the anatomy or neuroanatomy of the animal (e.g., precision of neuromuscular control, complexity of biosynthesis), and thus it must be considered in relation to the available range of possible signals. The particular modality(ies) involved, and signals used, depend(s) partly on evolutionary accident, but also on the nature of the information communicated (Marler, 1961).

Description of communication involves abstraction of the common features of recurring situations; Altmann (1965, p. 492) expresses this clearly :

Like other problems in classification, categorizing the units of social behaviour involves two major problems: when to split and when to lump. If one’s goal is to draw up an exclusive and exhaustive classification of the animals’ repertoire of socially significant behaviour patterns, then these units of behaviour are not arbitrarily chosen. On the contrary, they can be empirically determined. One divides up the continuum of action wherever the animals do. If the resulting recombination units are themselves communicative, that is, if they affect the behaviour of other members of the social group, then they are social messages. Thus, the splitting and lumping that one does is, ideally, a reflection of the splitting and lumping that the animals do. In this sense, then, there are natural units of social behaviour.

The ways of describing the behavior in question are clearly very important. As Cane (1961) indicates, there must be sufficient measures and their dependence or independence must be known [see also Altmann (1965) and the preceding chapter]. Measures of redundancy and information, as they are used by communication engineers, are, of themselves, of small value in studies of animal communication owing to the problem of describing the syntactic structure of animal communication (Smith, 1965). It is a task of misleading difficulty to determine information content and to incorporate the role of context or noise, for example. However, the advantages of “engineering information” analysis are that it is possible to specify the location of errors in the system and to compare quantitatively the communication in different modalities (Wilson, 1965; Haldane and Spurway, 1954; Moles, in this volume).

When armed with some knowledge of our subjects, as gleaned from naturalistic observations, we can turn our attention to empirically unraveling some of the complexities of the situation. Experimental studies can be directed toward the situations that elicit signals, the signals themselves, or to the responses to the signals. Clearly, the different types of study are not independent. Indeed, a major restriction on many experimental studies lies in the predominantly stochastic nature of much animal communication. This is particularly true in situations where movements are involved. In experiments one may find that the variables are being controlled at the price of distorting the interactions that comprise the communication. In these cases, the change in behavior that indicates that communication has occurred is itself communicatory.

Alterations in the physiological state of the communicator can be induced experimentally, as by injecting hormones, and subsequent changes in behavior can be observed. Interpretations of such observations must be cautious, since the changes are likely to involve shifts in several thresholds. Similarly, the immediate context can be altered: the composition of a social group can be changed or an individual can be moved; for example, a territorial robin will behave aggressively to a caged robin only when the cage is placed in its territory (Lack, 1943). The gradation of signals reflecting changes in that situation, e.g., alarm calls in response to increasing fear, needs to be investigated under controlled conditions. It is clear that these experiments are really but refinements of those sorts of observations described in the preceding chapter. Indeed, it is only rarely possible to distinguish clearly and absolutely the experimental and observational methods of study.

If we desire to investigate the repertoire of signals to which an animal responds, one technique is to present a series of lures, dummies, or other experimental devices, and to record the responses to them. However, this approach may not be very profitable unless it is accompanied or preceded by studies of sensory processes, psychophysical determinations of thresholds, etc. A familiar cautionary tale on the Umwelt of other animals is contained in the realization that crickets (Orthoptera) respond to the pulse pattern of the song of another cricket, whereas the frequency of the sound is what is most obvious to the human listener. When the Umwelt of the animal has been defined, the use of lures has proved very valuable, indeed. For example, analyses of the components of signals by means of dummies have shown that sometimes a complete response is elicited by one specialized structure alone [e.g., the red breast feathers of the European robin (Erithacus rubecula), but note that Lack (1943) discovered that these feathers must be in the territory of the robin]; in other cases the display or structure is apparently meaningless in the absence of the whole animal [e.g., facial movements of rhesus monkeys (Hinde, 1964)], again illustrating the arbitrary distinction between signal and context. The advantage of experimental studies with dummies is that they permit elaborate signals and sequences of behavior to be broken down into simpie components, e.g., the characters of a fish-like model that elicit courtship or aggression from a male stickleback in his territory (Tinbergen, 1951). On the other hand, analyses of complex signals may be influenced by their most evident context and meaning; for example, Bremond (1965) has made extensive playback experiments with the song of Erithacus rubecula, altering the experimental song in a variety of ways and thereby determining the significant characters of the song when apparently used as a territorial threat. However, it is not reasonable to assume that the same characters are significant to the mate. The experimenter must beware of generalizing on the basis of an analysis limited to a single type of situation. Dummies, playback experiments, and similar devices are also particularly prone to failure where movements are an integral part of the signal. In this case it may be profitable to disguise living animals; a pioneer in this technique was Cinat Tomson (quoted by Marler, 1959), who studied mate choice in female budgerigars by painting individuals to resemble “supernormal” males.

EVOLUTIONARY ORIGIN OF THE COMMUNICATION SYSTEM

As indicated by Wenner (this volume), honey bees can communicate the relative distance, direction, and quality of a food source. Who can doubt the advantage accruing to a colony capable of such communication? However, in other instances the value of communication may not be so obvious, and it is necessary that any study of communication show that the activity labeled “communication” does possess selective value. This demonstration is not always simply achieved. The function of the spring song among passerine birds is probably indeed to mark territorial boundaries and announce the availability of an unmated male. Yet, who can be certain that nonacoustic elements are not of equal, or even greater, importance in this instance of communication? The obvious experiment is to render mute territorial males and to compare their breeding success (the ultimate criterion) with that of their unimpaired neighbors. We do not know of this having been done. Until it has been, the various studies (e.g., Weeden and Falls, 1959) demonstrating discrimination between songs of near and distant neighbors, although excellent in themselves, offer only presumptive evidence for the functional significance of song.

Natural selection operates only upon the reproductive units, i.e., the individual and entire animal. Yet, in the social insects, in which communication is particularly well developed in all individuals, reproduction is relegated to but a small proportion of the total population. Since communication is but one aspect of social behavior, W. D. Hamilton’s (1964) attempts to develop a theory of the genetic evolution of social behavior are pertinent. Hamilton points out that behavior which benefits another more than it does the behaving individual (i.e., altruism) may still be favored provided the benefited individuals are related to the “altruist.” Specifically, if the gain in fitness to a relative of degree r is K times the disadvantage to the altruist, K must be greater than 1/r. Evolutionary studies of communication should exploit these ideas of Hamilton, elaborations of the earlier work of R. A. Fisher (1930), and Haldane (1932).

To summarize the foregoing, it is necessary to develop experimental techniques (1) for establishing that a given signal system is subject to selection for communication and (2) to explain the genetic mechanisms that permit development and maintenance of the relevant traits, particularly where reproduction is limited to one caste.

There are two further evolutionary problems. As Hinde (1964) has indicated, natural selection does more than merely lead to the establishment of communication. Selection will also shape the character of the signal and of the response. It is necessary to develop experimental techniques for ascertaining the nature of the selective influence. For example, Marler (1959) has compared calls given by a variety of passerine birds in response to the overflight of a hawk. These calls are strikingly similar when viewed audiospectrographically, and they also have a psychophysical similarity in being very difficult to localize. In contrast, the flight calls of these same birds are relatively easy to localize. Thus, we assume that these particular signals have evolved so as to be either maximally or minimally conspicuous, depending on whether their purpose is best served by their being readily localized or not. Selection may also act to enhance or to diminish specific distinctiveness: island bird species with relatively few competing congeners have a reduced specific distinctiveness compared with their mainland counterparts (Thorpe, 1961).

Hinde (op. cit.) also considers the nature of selective pressures on responsiveness. Sometimes the response system of one individual is attuned to a restricted cue: the red patch of the English robin in territorial conflicts between male robins, for instance, referred to earlier. Other times, a more complex or even multimodal signal must be received before a response can be expected, as in butterfly courtship or rhesus monkey communication. Obviously, experimental proof of the role of selection in shaping particular signals or the responses to them can be expected but rarely. Comparative studies like those alluded to above (Hinde, 1964 and 1959) have provided the basis for deductive inferences concerning the evolution of responses.

In the preceding paragraphs we considered the manner in which evolutionary forces act upon communication systems; now we shall consider the techniques for studying the course of evolution, i.e., the history of the communication system of particular species. We owe to Niko Tinbergen (1962) the clearest statement of the methods for historical, or evolutionary, studies of communication. Following him, one first establishes the degree to which a pair or group of species are related to one another, this determination being largely based on character affinities in accord with conventional, taxonomie procedure. If the similarities imply common ancestry, the behavior of the ancestral form is reconstructed. The reconstruction is based upon homology, two criteria for which are “extreme similarity and widespread occurrence” (p. 3). Then, the “origin” of the movement(s) in question is sought. It is assumed that the most aberrant species in a group, or the species living in the most “specialized niche,” are least likely to be most similar to the ancestral pattern, and the converse. As Tinbergen puts it, “These arguments are all full of pitfalls” (p. 3). We cannot unreservedly accept his further conclusion, however, that “when the various criteria are applied together and all point in the same direction, they carry conviction” (loc. cit.). The addition of uncertainties does not increase the reliability of the sum, and the various criteria are often not demonstrably independent. The fundamental difficulty lies in the fact that the most satisfactory evidence for a given line of descent is that most rare of phenomena, a statigraphically intact paleontological series. Physical similarities in extant descendants of extinct ancestors are reliable indicators of relationship only where the genetic basis of the characters in question is known, as well as the susceptibility of the phenotype to environmental influences [e.g., the scutes of snakes are frequently used as a taxonomie character, but experiments have shown them to vary according to the temperature at which the embryo develops (J. R. Bailey, personal communication)]. Further, serologic, embryologie, or other similarities should not be considered as independent of anatomic similarities: a similar embryologie history implies similarity both in biochemistry and gross morphology. Finally, no satisfactory a priori basis for accepting the “rules of specialization and aberrance” (above) has yet been advanced. Until “most aberrant ecology” and “most specialized form” have been operationally defined, it is unprofitable to toy with these terms or to hope that experimental evidence on serologic and other affinities will be helpful.

However, although we are skeptical of the construction of phylogenies, especially of phylogenetic trees, it would be most unwise to condemn thereby all comparative studies. It is clear that behavioral comparisons can often be a fruitful source of understanding of origins and mechanisms—e.g., ritualized signals—even though a rigorous experimental proof may be unattainable.

In summary, an empiricist interested in the history of communication must first devise reliable indices of affinity among the species he is studying. Genetic tests and hybridization studies probably offer the most hope. The conventional morphological approach of the systematist is useful if (1) the genetic basis (and susceptibility to environmental influences) of the characters used is first established and thus (2) the independence of the characters is documented.

Once the phylogenetic relation becomes known, it is necessary to find experimental support for the deductions based upon the form-function relationship that Tinbergen uses. The facile assumptions that underlie so much of ethology are no substitute for an actual experimental demonstration that, e.g., the changes in the shape, size, or color of a gull’s bill actually influence either the gull’s own behavior or that of its companions and chicks. Tinbergen’s (1965) study of eggshell removal by gulls in their nest, while not an instance of communication, is worth noting as a particularly cogent example of such a demonstration: it was empirically demonstrated that nests from which egg shells were not removed did suffer heavier predation!

ONTOGENETIC DEVELOPMENT OF COMMUNICATION

Historically, behavior has long been thought to fall into one of two categories: on the one hand, reasoned or learned behavior, on the other, instinctive behavior. Although some biologists have believed these two categories to be distinct, most have followed the lead of Darwin and Romanes, who held that learned and instinctive acts could grade into one another both phylogenetically and ontogenetically. Darwin made this explicit in his suggestion that instincts could arise from “lapsing intelligence,” i.e., from the frequent and eventually unconscious repetition of an originally learned habit. “Lapsing intelligence” allows for a transition from learned to instinctive behavior, at least at the ontogenetic level. Since Darwin also believed that acquired characters (including behavior) could be inherited, he effectively bridged the gap between ontogeny and phylogeny as well (Klopfer and Hailman, 1967).

Modern ethology, in contrast, tended, at least in its early years, to sharpen rather than blur the instinctive-learned dichotomy, though in a sophisticated manner. In essence, ethology has considered instinctive behavior to meet two criteria: (1) it is a highly stereotyped, species-characteristic motor sequence which (2) depends upon a particular “drive” or central mechanism [generally described in terms of a hydraulic model (Lorenz, 1950)]. However, there is no a priori reason to believe that the descriptive element (1) and the functional element (2) are concordant. That is, a motor pattern may be highly stereotyped, transmitted in Mendelian fashion, and have all the other hallmarks of “instinct” as descriptively recognized, yet not depend on a mechanism such as that originally implied by the hydraulic model. Alternatively, behavioral mechanisms comparable to those embodied in Lorenz’s model may exist even in cases where the motor response itself is quite variable’. It would thus seem preferable to regard the term “instinct” as applicable, descriptively, to examples of species-characteristic behavior without regard to ontogeny or underlying mechanism. This allows for the possibility that any act above the reflex in neural complexity is an amalgam of events and elements whose properties and form are fixed at different stages of development and as a consequence of a variety of mechanisms. It allows for the possibility that either (or both or neither) a unitary or dichotomous concept of behavior is appropriate. We should remove all need for students of communication to analyze their subject in accordance with any particular behavioral schema. The questions they should rather ask are these: Are the elements (pragmatics and syntactics) of a given communicative system fixed or variable? Within what limits does variation occur? Do the variations persist through successive generations (are there traditions)? What selective factors lead to the origin and maintenance of variations or, conversely, to their elimination? These questions can be expected to yield to an empirical approach, as illustrated, for example, by the studies of bird song by Marier and his students (see, e.g., Chapter 15).

In studies of the ontogeny of communication, great simplification (oversimplification?) becomes a practical necessity. In the interests of this economy, we recognize three important variables. It must be noted that these apply equally to the communicator and the recipient. Most studies have focused upon the communicator, but note Wenner’s work with bees (Chapter 11), in which the behavior of the recipient was also studied. There are three variables, and they are discussed in the following sections.

Degree of Isolation

Isolation may be merely from adults of the species, or it may include separation from peers. Such an isolate, of whatever degree, is known as a Kaspar Hauser, after a German youth alleged to have been reared alone in a cellar. Studies imposing conditions of isolation may present evidence on the role of certain kinds of learning, provided the conditions of rearing are not such as to create pathologies. The most extreme condition of isolation involves elimination of self-stimulatory effects by deafferentation, e.g., by deafening, thus precluding perception of one’s own voice. This kind of treatment cannot be extended to all modalities without creating obvious behavioral disturbances in addition to those relating to communication. The same difficulties are found with experiments in which an effort is made to limit or enrich the nature of the stimulation provided in particular modalities. Such studies are necessary to determine what factors fix the extent of an organism’s repertoire. However, restricting the vision of a cat (by rearing in the dark) causes degeneration of certain retinal elements and deficiencies in vision that are not limited to visual communication (Weiskrantz, 1958). Finally, it is necessary to distinguish between the effects of “no stimulation” and “no patterned stimulation.”

Timing of Isolation

A given experience with peers or parents may depend for its effect upon the age or developmental stages at which it occurs. The phenomenon of imprinting represents a paradigm of learning that, if it occurs at all, must occur within a relatively brief period in the life of the individual. That temporal considerations are important to the design of studies of the ontogeny of communication will be evident from the other examples cited below.

Threshold and Context

Differences in the responses of isolate-reared or normally reared young could well be due to differences in thresholds for, e.g., fear or aggression. Such motivational differences, in turn, might alter the subjective context within which signals are perceived. This particular variable has been routinely neglected in work with isolates (Kaspar Hauser animals), and it appears to us to deserve considerable attention.

Most signals are given in the correct context without previous encounters with other members of the species. A dog raised in complete isolation from its kind will give dog-characteristic threats when confronted by a rival. Details of its performance, orientation, and eliciting situation may well change with experience, but the main features seem to be independent of the environment in which the animal is raised. Sound signals appear to provide exceptions to this generalization, a difference that further justifies experiments upon context.

The most detailed studies of the acquisition of a set of signals have been directed to bird songs. It has been shown in a wild population that males may differ from each other in their songs and that such differences may be recognized by other males (e.g., Weeden and Falls, 1959). Modifications of auditory experience both by isolation and by addition of atypical sounds have revealed a wide array of patterns of development. For example, Konishi (1963) found that all vocal signals made by the domestic fowl develop almost normally in deafened birds. At the other՝”end of the “open-closed” developmental spectrum, Nicolai (1964) has shown that certain African ploceid brood parasites learn the song patterns of their host completely. The chaffinch is an interesting intermediate case; some characters of its song are independent of particular auditory experience, whereas others are learned during certain critical periods [Thorpe (1961) and Marler (1961) review much of the literature]. Under all conditions of rearing the frequency range and temporal patterning of the notes is approximately the same, but the details of these notes and the division of the song into its usual three parts is dependent on hearing the normal song before the first winter; i.e., there is a predisposition to incorporate certain kinds of sounds into the song. Further characters of the song are learned during territorial encounters with other males during the following spring—and they are thereafter fixed for the life of the bird.

Nicolai (1959) has found a different mechanism in the bullfinch (Pyrrhula pyrrhula). In this species the young males learn only sounds given by a bird (or in exceptional circumstances a mammal) with which an emotional bond is established in youth. In nature the young male usually learns the song and calls of his father, but abnormal rearing conditions can result in the learning of abnormal songs which are passed on to succeeding generations without reversion to the species-characteristic type. However, the singing of a song is only a part of the communication; the role of experience in the use of the song in the wild has not been established, nor has the effect of isola tion upon the recipient been investigated. Much experimental work remains to be attempted in the development of communication by bird song. The ontogeny of other communication systems is even less well investigated.

In summary, experimental considerations of communication need to take into account the character of the communication system (ineluding signal, context, and meaning), its evolutionary origin, and ontogeny. Experimental methods have been most successfully brought to bear on the first of these three areas and least successfully (if at all) on the second (evolutionary) studies. We believe that an experimental approach can yet provide insights into the evolutionary course. The study of ontogeny of communication is surely still in its infancy.

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